In the human adult, the ileum is typically about 12 feet long and the entire small intestine is approximately 21 feet long. In contrast, the human large intestine has a length of about 5 feet. The proportional relationships between the intestinal sections of other species vary, however, generally reflecting differences in diet. For example, the large intestine is proportionally much more dominant among nonhuman primates, such as orangutans and gorillas. These animals chiefly depend on plants for their nutrients, while humans generally have an omnivorous diet.

A rat ileum tissue section (shown above) was stained with Oregon Green 488 conjugated to wheat germ agglutinin, a plant-derived lectin that targets the Golgi apparatus, as well as Alexa Fluor 568 conjugated to phalloidin for cytoskeletal actin. Cell nuclei were labeled with Hoechst 33342. Images were recorded in grayscale with a 12-bit digital camera coupled to a Nikon Eclipse 80i microscope equipped with bandpass emission fluorescence filter optical blocks. During the processing stage, individual image channels were pseudocolored with RGB values corresponding to each of the fluorophore emission spectral profiles.